The present invention relates generally to the field of GPS receivers and specifically to a GPS receiver integrated with a wireless communications mobile terminal.
Wireless mobile terminals, utilized in mobile communications systems, are increasingly being integrated with a broad array of consumer electronics devices to expand their functionality and offer a greater range of services to the user. One such consumer electronics device is a geographic position estimator, and in particular, one that derives position estimates from the reception and decoding of signals broadcast by navigation satellites. The predominant example of such a satellite navigation system in the U.S. is the Global Positioning System (GPS).
The provision of a geographic position estimator in a wireless communications mobile terminal provides several benefits and enables the delivery of advanced services to the user. For example, the geographic position of a mobile terminal may be supplied to emergency services operators whenever the user places an emergency call using the mobile terminal. This ability is required by current and proposed FCC regulations. A user may optionally desire to transmit position information to the called party in other situations, for example, to obtain directions, or to update his position as part of a job requirement. A position estimator also enables the delivery of a variety of position-based information services, such as maps showing a user's location and directions to a destination, a listing of hotels, restaurants, or other businesses offering particular services within the user's immediate area, and the like. Additionally, knowledge of the mobile terminal's geographic location enables position-targeted advertising, such as for example, transmitting to the mobile terminal an electronic coupon redeemable at a business establishment as the user passes within walking distance of the business.
The GPS navigation system operates by simultaneously calculating a receiver's range to several GPS satellites and calculating the receiver's position based on those ranges (a process known generally as trilateration). To determine a receiver's position on the surface of the earth (i.e., in three dimensions), the range to at least three separate satellites must be determined. Each range is calculated from the propagation time of a signal transmitted from the satellite to the receiver, and the signal's known speed (which is, for GPS radio signals, the speed of light in a vacuum, and somewhat less through the ionosphere and atmosphere). The greatest source of error in the range calculation is inaccurate measurement of the signal propagation time, due to the inherent difficulty of accurately synchronizing time measurement between the precision atomic clocks located on the GPS satellites and the local clock at the receiver. This time synchronization problem is resolved by calculating the range to a fourth GPS satellite. Thus, a receiver's position estimate is derived by resolving four unknown quantities (three locational coordinates and time) using the range and position information from four satellites.
GPS satellites orbit at approximately 11,000 miles altitude, and consequently the strength of their broadcast signals received at the earth's surface is extremely low. Various signal reception, amplification, filtering and multi-path resolution techniques to accurately and reliably receive and interpret GPS signals are known in the art and commonly employed in stand-alone GPS receivers. A significant issue facing the integration of a GPS receiver into a wireless communications system mobile terminal, however, is that radio interference caused by the mobile terminal's transmission of radio signals as part of its communication with the mobile wireless communications system hampers the reception of GPS signals.